1. Field of the Invention
This invention relates to a piezoelectric device of, e.g., the energy trapping type, utilizing thickness expansion vibration or thickness shear vibration, such as a vibrator, filter, trap or discriminator, and particularly it relates to a chip type piezoelectric device having an element body with an integrally sintered piezoelectric ceramic structure.
2. Description of the Prior Art
When it is desired to obtain a resonant frequency of, e.g., 10 MHz using a piezoelectric element in the form of a single plate of piezoelectric ceramic material having electrodes formed on oppposite surfaces thereof, the thickness of such single plate would have to be 200 .mu.m or thereabouts. Therefore, the mechanical strength would be low and the element or device, as it is, would be difficult to handle. As an approach to solving these problems it is the usual practice to employ resin dipping or packaging, but in each case lead terminals must be attached to the respective electrodes, adding to the size of the device; particularly, the height of a device in its mounted position increases.
To attempt to solve the prior art problem described above, a chip type piezoelectric device of the construction shown in FIG. 7 has been proposed (for example, Japanese Patent Application Laying-Open Gazette No. 172821/1984). The piezoelectric device shown in FIG. 7 has an element body 1 formed of cofired ceramic material and internally provided with a pair of excitation electrodes 2 and 3 opposed to each other. The excitation electrodes 2 and 3 are electrically connected to external electrodes 4 and 5, respectively, formed on the exterior of the element body 1.
The ceramic material forming the element body 1 has piezoelectricity imparted thereto so that when a driving or excitation voltage is applied thereto by the external electrodes 4 and 5, the portion defined between the excitation electrodes 2 and 3 vibrates at a predetermined frequency. Cavities 6 and 7 are formed on the respective sides of the excitation electrodes 2 and 3 opposite to the vibrating portion defined between the electrodes 2 and 3, thereby to make the vibrating portion vibrate independently of its surrounding areas.
According to the chip type piezoelectric device as shown in FIG. 7, since the thickness of the element body 1 can be increased, the mechanical strength of the element body 1 can be sufficiently high. Further, since the presence of the cavities 6 and 7 prevents vibration in regions between the respective cavities 6 and 7 and the respective outer surfaces of the element body 1, it is possible to mount the device directly on a printed circuit board or the like. Therefore, the device can be easily handled while being reduced in size, and its height as mounted can be reduced as well.
However, the chip type piezoelectric device shown in FIG. 7 has a vital drawback: The resonant frequency of said chip type piezoelectric device is determined by the distance between the excitation electrodes 2 and 3 forming a pair, such distance being already determined at the firing stage performed to obtain the element body 1. Therefore, once the chip type piezoelectric device has been obtained, it is impossible to make any adjustment, e.g., a fine adjustment, of the resonant frequency thereof.